For example, FIG. 5 of Patent Literature 1 illustrates an optical adapter for connecting a pair of optical connectors. The optical adapter has on both sides thereof a pair of attachment holes into which the optical connectors are attached. On inner peripheries of the attachment holes, there are provided a split sleeve and a sleeve holder for holding the split sleeve from an outer periphery thereof. Respective distal end portions of ferrules of the optical connectors are inserted from both sides along an inner periphery of the split sleeve, and the distal end portions of the ferrules are brought into contact with each other inside the inner periphery thereof.
FIG. 8 illustrates an optical-fiber connection unit 100 including an optical connector 110 and an optical adapter 120 having a similar structure to that disclosed in Patent Literature 1. The optical connector 110 includes a ferrule 111 having an inner periphery inside which an optical fiber (not shown) is allowed to pass, and a housing 112 for retaining the ferrule 111 from an outer periphery thereof. The optical adapter 120 includes the following: a casing 121 having attachment holes 121a1 and 121b1 for the optical connector 110, the attachment holes being opened on both sides, in an axial direction; a split sleeve 122 having an inner periphery along which a distal end portion of the ferrule 111 is inserted; and a sleeve holder 123 for retaining the split sleeve 122 from an outer periphery thereof. The casing 121 is constituted by two members 121a and 121b formed by being divided substantially at an axial central portion and bonded to each other. Note that, the axial direction represents a direction in which the optical fiber is passed in a state in which the optical connector 110 and the optical adapter 120 are unitized (hereinafter, the same applies), and hence a lateral direction in FIG. 8 is the axial direction.
The split sleeve 122 is provided for performing centering of the pair of ferrules inserted from both the sides in the axial direction. In the state illustrated in FIG. 8, the distal end portion of the ferrule 111 of the optical connector 110 is inserted from one side in the axial direction (right side in FIG. 8). In this state, a distal end portion of a ferrule of another optical connector (not shown) is inserted from another side in the axial direction (left side in FIG. 8) of the split sleeve 122. Then, distal end portions of both the ferrules are hit against each other in a state of being subjected to centering on the inner periphery of the split sleeve 122.